A field guide to the ways reasoning breaks
Broken structure
Also called Inclusive or error
Assuming that because one option in an or statement is true, the other must be false.
She is in sales or marketing; she is in sales, so she is not in marketing.
Ask whether the or was meant as exclusive. Usually both can hold.
The name describes the move: one half of an 'or' statement is affirmed, and from that the other half is denied. The error comes from reading 'or' as exclusive — at most one option true — when ordinary usage is often inclusive, and both can hold at once.
The outage was caused by the database or the cache; we confirmed the database, so the cache is fine.
He is on leave for family reasons or burnout; HR confirmed family reasons, so it is not burnout.
The delay came from the vendor or from our own approvals; the vendor was late, so our approvals are cleared.
Users churn because of price or features; the survey says price, so the feature roadmap is not the problem.